In factory installations where there is a large volume of air flow, for example in ventilating systems for plants, drying rooms, motor housings and the like, the air supply is normally taken from within the factory or else through special fresh-air inlet ducts. In ventilating systems where the air is conditioned, the conditioning apparatus is effective to remove the dust and other foreign particles which are taken in with the fresh air from the factory or outside. However, where the ventilating air is not conditioned, there is frequently no attempt to filter the air. In motor housings, the presence of foreign particles in the air stream may lead to premature failure of the motors or may cause arcing or other dangerous conditions within the housing. Normally the inlet to the housing is provided with a grille which prevents trash or other large debris from entering the system, but such grilles do not prevent entry of dust or small particles. It has been the practice where dust presents a problem to simply use fiberglass furnace replaceable frame filters in the air inlet in order to remove the particulate matter which would otherwise enter the housing through the inlet.
Although the cost of fiberglass filters is minimal, in heavily dust-laden atmospheres, the filters tend to become clogged with dust particles, particularly where a humidity factor causes agglomeration of the dust particles in the filter medium. As a result there must be frequent servicing of the filters, normally entailing a brief shutdown of the equipment while the individual filters are removed and replaced with fresh filters. Because of the nuisance involved in exchanging filters, the task is often delayed or deferred to a point where the effectiveness of the filter is completely dissipated by reason of its being clogged with particles. If the deferment of servicing the filters is excessive, the motors or other equipment which is being ventilated by the air flow may be subject to premature failure.